1. Field of the Invention
The present invention relates to a method of manufacturing a thin-film field-emission electron source and, more particularly, to a method of manufacturing a thin-film field-emission electron source having a tip portion of an electron emitting area which employs evaporation and photoetching.
2. Brief Description of the Prior Art
In general, a prior-art field-emission electron source is used in a construction in which a substance to emit electrons is formed into a sharp needle-like shape and is made a cathode, while an electrode plate for acceleration is provided on the outside, so as to concentrate the electric field on the tip of the needle.
As the material of the needle-shaped cathode, a single crystal or polycrystal of tungsten is mainly used. Recently, borides such as LaB.sub.6 have also come into use.
Such a field-emission electron source, however, has the disadvantages of (1) the necessity of a superhigh vacuum (about 10.sup.-.sup.10 Torr), (2) the necessity for a high voltage power source (several tens kV) and (3) instability in the emission current. Therefore, field emission is not widely applied as compared with the thermionic emission etc.
As a field-emission electron source free from the disadvantages, there has recently been proposed a thin-film field-emission electron source which has a sandwich structure of a substrate-metallic film-insulating film-metallic film and which has a minute cavity and a field-emitting cone within the minute cavity. Such a thin-film field-emission electron source operates at a low voltage. Since the emission source is well shielded and the concentrated electric field part is confined within the cavity, its stability increases. It is also considered that the degree of vacuum may be lower than in the prior art.
Regarding the manufacture of such a thin-film electron source in which the emitter and the accelerating anode are thin films, two methods to be explained hereunder are known.
The first method includes the step of evaporating, on a substrate of sapphire or the like, three layered films of metal - insulator - metal such as Mo - Al.sub.2 O.sub.3 - Mo. A minute cavity penetrating through the second and third layers is formed by a suitable mask evaporation process and/or etching process. In order to make a cathode with a tip in the cavity, two materials are respectively evaporated by oblique evaporation and normal evaporation. As the opening of the cavity gradually closes by oblique evaporation, the tip portion to be the emitter is created within the cavity by normal evaporation. Finally, only the material deposited by oblique evaporation is selectively dissolved and removed. Thus, an electron source is constructed.
The second method resembles the first method, but it differs in the manner of producing the tip portion. By utilizing the action of the first layer material or an additive material thinly covered on the first layer beforehand, the tip portion is precipitated or crystal-grown within the cavity by heat treatment. Although the theory underlying the method is partially unsolved in principle and is not clear, it can be employed on some types of materials. The method has the merit that a plurality of tip portions can also be formed within the cavity.
In the above two methods, the second has the greatest difficulty in that the most excellent material for the electron source with which electric fields are concentrated cannot be freely selected and used for the material of the tip portion. The materials which have been proven to be capable of forming the tip portion are of a small number.
On the other hand, the first method is not subject to the foregoing restriction concerning the material of the tip portion as in the second method, and hence, it can be said to be excellent. It has, accordingly, been considered that this method is an excellent manufacturing method for a known thin-film field-emission electron source.
In this method, however, the simultaneous evaporations of normal evaporation and oblique evaporation employed for constructing the tip portion within the cavity require an extremely high degree of precision in the method of manufacturing thin films. In particular, the necessity for the precise control of both the evaporations creates difficulty in the manufacture.
The enhancement of the manufactural yield in the first method is, therefore, subject to limitations. Where it is intended to distribute a large number of electron sources in a large area, manufacture is extremely difficult, even if possible in principle.